At present, the engines used for helicopters (sometimes referred to below as “aircraft”) are usually of the “free turbine” type (also known as the “power wheel” type) where power is taken from a low-pressure stage of the turbine, which stage is mechanically independent of the compressor installation of the high-pressure stage of the turbine.
In principle, a turboshaft engine has a speed of rotation lying in the range 30,000 revolutions per minute (rpm) to 50,000 rpm, with a speed of only about 6,000 rpm at the outlet from the incorporated gearbox that is generally associated therewith.
Unfortunately, the main rotor of a helicopter has a speed of rotation lying in the range 300 rpm to 400 rpm, such that a special speed-reducing gearbox is essential on all helicopters. That is why a main gearbox (MGB) is always installed between the turboshaft engine(s) and the main rotor: which MGB is naturally associated with transmission means.
In most cases, the helicopter is also provided with a tail rotor for controlling yaw movements of the aircraft. Such a helicopter then has a transmission shaft between a special take-off of the MGB and a rear gearbox providing angle transmission and a reduction in speed to about 2,000 rpm, for example, to deliver power to the tail rotor.
It is important to observe that terms such as “front” and “rear” relating to an element of a helicopter designate respectively the portion of that element situated towards the cockpit (i.e. at the leading end of the aircraft), and the portion situated towards the tail boom and the tail rotor.
Under such conditions, the invention relates to a power plant having one or more engines on a helicopter, located behind the MGB, even though the architecture of such engines predisposes them for being installed in front of the MGB. Like the configuration of the Makila® 1A or 1A1 turboshaft engine developed by the supplier Turbomeca and fitted in particular to the helicopter known under the trademark Super Puma® in the name of the Applicant, it must be understood that such an engine, disposed in front of the MGB presents successively from the front towards the rear of the helicopter the following members:                a gas generator comprising in succession:                    a short air inlet;            a three-stage axial compressor connected to a rear centrifugal compressor by the shaft of the gas generator;            a combustion chamber; and            a two-stage turbine of the gas generator;                        a free turbine comprising a two-stage working turbine driving towards the rear a power transmission shaft or turbine drive shaft connected to specific inlet of the MGB; and        a hot gas ejection nozzle directed laterally and outwards from the engine compartment.        
In a Super Puma® helicopter, the power plant comprises two Makila® 1A or 1A1 engines, each being installed in an individual compartment that is fireproof, ventilated, and drained.
More generally, the solutions that have been implemented in the past correspond to two main architectures, namely:                the configuration of the first solution comprises one or more engines mounted in front of the MGB like the installation of the Makila® 1A or 1A1 engine on the Super Puma® helicopter as described above; or        the configuration of the second solution comprises one or more engines behind the MGB, which engines are specially designed for a disposition in which there are to be found, after an MGB, and in succession from the front towards the rear of the rotary wing aircraft:        the turbine drive shaft driving the MGB, which shaft:                    either goes towards the front of the helicopter through the air inlet, passing through the gas generator (and its compressor) and is driven by the free turbine, as applies for example in the L.T.S. engine from the supplier Lycoming that is fitted to the Ecureuil® AS 350® or AS 355® helicopters of the Applicant;            or else lies parallel to the engine, but outside it, and is driven by the free turbine, as applies for example to the Arriel® engine that can also be fitted as an alternative solution to Ecureuil® AS 350® or AS 355® helicopters;            the gas generator;            the free turbine; and            the nozzle for exhausting hot gas, constituting the element of the power plant that is situated furthest towards the rear of the helicopter.                        
Unfortunately, present certification regulations no longer enable a new civilian helicopter to be certified without a significant increase in weight if the engine(s) is/are installed in front of the MGB, i.e. with the configuration of the first solution described above. Under such circumstances, it is now required to shield the engine in order to protect the flight controls (servo-controls, swashplates, blade pitch links, . . . ) from the possibility of the turbine bursting, these flight controls being in the immediate proximity of the turbine and being distributed in particular around the rotor shaft as driven by the MGB.
It is important to observe that that is why engine manufacturers have developed engines complying with the second solution described above. As already mentioned, it will be understood that those engines correspond to definitions specially adapted for installing engines behind an MGB so that the air inlet is towards the front of the helicopter and the nozzle towards the rear. The examples relating to the L.T.S. engine with a turbine drive shaft passing through the gas generator (and the compressor) or relating to the Arriel® engine with a turbine drive shaft parallel to and outside the engine clearly illustrate the special functional arrangements that are needed in association with that second solution.
Engines suitable for that second solution are relatively recent, but they are more expensive than those suitable for the first solution, because of the particular technical problems inherent to their special disposition behind an MGB.
Unfortunately, the cost of a power plant becomes even greater for a twin-engine helicopter that is intended to be inexpensive to purchase.
A third solution consists in fitting a helicopter with engines located behind the MGB, but using engines that are theoretically intended to be installed in front of the MGB.
One such solution has been put into practice in the SA 321 Super Frelon® helicopter of the Applicant.
In that configuration, the helicopter is provided with three Turmo® III C3 engines from the supplier Turbomeca, two of them being disposed side by side using the first solution, while the third engine is located behind the MGB, the other way round to the first two engines, i.e. in the following configuration:                a hot gas exhaust nozzle behind the MGB;        a free turbine drive shaft connecting it to the rear of the MGB;        a gas generator; and        an air inlet, constituting the rearmost element of the power plant.        
It should be observed that the positioning of the third engine presents a drawback due to the air inlet being located behind the nozzle. As a result, particularly in forward flight, it will readily be understood that at least some of the hot gas coming from the nozzle can be fed into the air inlet. This constitutes a phenomenon known as “recirculation” and that has a severe effect on performance, i.e. the power of an engine suffering therefrom. Naturally, this loss of power, which is acceptable in a helicopter with surplus power such as the Super Frelon®, is no longer acceptable for a single-engine or twin-engine helicopter, particularly if the helicopter is to be inexpensive to purchase.
In this context, it should be observed that an inverse configuration in the manner of the third engine of the Super Frelon® is to be found in document GB-0 864 540. In that configuration, the air inlet is located in the wing of an airplane, above the nozzle situated upstream therefrom. In that configuration likewise there is a possibility of some of the hot gas penetrating into the air inlet, particularly when the wing is at an angle of incidence, and in spite of the presence of the propeller that provides a slipstream going from front to rear.